1. Field of Invention
This invention relates generally to FM "chirped" laser radar systems and, more particularly, to a pulse compression signal processor which utilizes identical sound acoustic wave (SAW) matched filters for processing both the up and down chirps of a bi-directional linear FM echo signal.
2. Discussion
Laser radar systems which employ an intensely focused beam of light to detect the presence, position and motion of objects have been used in numerous applications, especially in the radar communications and measurement fields. Militarily, these systems have been implemented in conjunction with cruise missile and tactical fighter technology wherein laser radar has provided functions such as obstacle avoidance and terrain following. These laser radar systems enable sophisticated target homing capabilities for accurately guiding a missile or plane toward a target by utilizing a distinguishing feature of that target.
Linear frequency modulated (LFM) "chirped" laser radar has proven to be particularly useful in these applications. A "chirped" laser radar system typically includes a continuous wave (CW) transmitter which emits laser light at a preselected center frequency f.sub.c. This emitted light is frequency modulated into linear "chirps" by passing it through an electro-optical device disposed within the cavity of the transmitter. The shape of modulated waveform can be varied somewhat in order to optimize trade-offs in efficiency, complexity and performance.
A bi-directional type of waveform, as shown in FIG. 1, is often used to minimize range/doppler ambiguity in the system as well as to maximize the system's ambiguous range. To create this waveform, the frequency of each transmitted pulse is first increased and then decreased at a constant rate. The frequency variation created is preferably linear and the frequency versus time characteristic of the signal is typically of a trapezoid pattern as shown by the solid line in FIG. 1. Each chirp in the signal includes an "up chirp" component wherein frequency is increasing and a "down chirp" component wherein the frequency is decreasing.
The transmitted chirped signal is directed toward a target and then reflected back therefrom, creating a return signal or "echo" associated with the target. The time taken by the transmitted signal to reach the target and return causes the return signal to be displaced in time with respect to the transmitted signal. The echo signal naturally retains the same increases and decreases in frequency it had as a transmitted signal. This is shown graphically in FIG. 1 wherein the solid line represents a transmitted signal Tx and the dashed line is used to indicate a corresponding return signal Rx. As illustrated in FIG. 1, both the transmitted and return signals have an up chirp where frequency is steadily increasing and a corresponding down chirp where frequency decreases.
The instantaneous frequency difference between the transmitted and return signals is indicated in FIG. 1 as f.sub.o. To obtain this frequency difference, the return signal may be compared to a reference signal which is typically a sample of the transmitted signal created by using a beamsplitter to retain a small portion of the transmitted beam. Properly scaled, this instantaneous frequency difference f.sub.o can be used to "demodulate" the return signal in order to ascertain certain information about the target and its location.
To obtain both long range detection capabilities and fine range resolution, extremely narrow pulses of exceptionally high peak power need to be transmitted. But, because there exist practical limitations on the level of peak power used, the laser radar system typically utilizes a relatively long coded pulse as the transmitted signal and then takes advantage of pulse compression of the return signal to obtain a narrower pulse. This enables achievement of the increased detection ability of a long pulse radar system while also retaining the range resolution capability of a narrow pulse system. Transmission of long pulses also permits a more efficient use of the average power capability of the radar without generating high peak power signals.
Pulse compression allows the transmission of modulated pulses of sufficient width to provide the average power necessary to illuminate targets, at a reasonable level of peak power. The received echoes are then compressed by decoding their modulation to obtain the range accuracy and resolution equivalent to that of a short pulse. This is accomplished by increasing the transmitted signal bandwidth by modulating the frequency of the carrier within the transmitted pulse. With a matched filter, the received long pulse can be compressed to a short pulse of duration equal to the inverse of the transmitted signal bandwidth.
The various methods of pulse compression are essentially matched filtering schemes in which the transmitted pulses are coded and the received pulses are passed through a filter whose time-frequency characteristic is the conjugate (opposite) of the coding. This function is usually performed within the signal processor used to process the received echo.
The matched filter introduces into the signal a time lag that is inversely proportional to frequency. As illustrated in FIG. 2, a filter for compressing an up chirp has a signal transit time which decreases linearly with increasing frequency, at exactly the same rate as the frequency of the echo increases. The trailing portions of an up chirp echo, being of a progressively higher frequency, take less time to pass through than the lower frequency leading portions, thereby causing successive portions to bunch together or compress. When a pulse has been compressed by the filter, its amplitude is much greater and its width is much less than when it entered. While the output echo may be only a fraction of the width of the received echo, it can have many times the peak power.
In laser radar systems this pulse compression is typically performed with a surface acoustic wave (SAW) device used as a matched filter. The signal transit time may either decrease with frequency as shown in FIG. 2, to compress an up chirp, or else increase with frequency for compressing a down chirp. A filter of the type shown in FIG. 2 is often referred to as a negative slope device and a filter having an opposite configuration in which frequency increases with transit time is referred to as a positive slope device.
In previous laser radar systems, individual SAW matched filters were uniquely developed for processing each of the up and down pulses of a bi-directional echo signal. A positive slope SAW matched filter was used to compress the down chirp and a negative slope SAW matched filter was used to compress the up chirp.
However, SAW devices have a bulk mode problem with long delays which is aggravated with high frequencies. In a positive slope device, the higher frequencies take the longer delay through the filter. The bulk mode problems generate spurious signals difficult to eliminate by filtering. The output is also often preceded and followed by sidelobes, an undesirable series of lesser pulses.
There is, therefore, a need for a LFM chirp laser radar signal processor which substantially reduces these problems and is able to compress both the up and down chirps with improved performance.